Composite materials are increasingly being used for the inner and outer skins of commercial aircrafts. In order to meet the commercial aeronautics industry demands for airworthiness and flight safety, these new composite material structure require the development of new inspection technologies. Conventional inspection techniques include a so called “coin tap” testing method where the inspection conductor taps the suspected areas lightly with a hard and blunt tool to obtain indications of the underlying structure from the sound of the tap. Other testing methods include thermographic testing, non-linear spectroscopy, X-radiography, eddy current measurements and ultrasonic waves. Among these inspection methods, the ultrasonic wave method is the most commonly used testing method for non-destructive inspection of aircrafts. However, ultrasonic wave testing methods are not suitable for composite structures where non-isotropic properties of the composite structure materials cause high attenuation due to absorption and scattering.
Recent advances in sonic techniques such as pitch-catch method and resonance methods are able to obtain high sensitive responses from aircraft composite structures employing an excitation frequency lower than 100 kHz. In the pitch-catch method, a Lamb wave for composite structure inspection is generated and received by two respective piezoelectric probes located at a distance from each other on the surface of the composite. The behaviour of the Lamb wave in terms of wave mode, frequency, velocity, and level of attenuation is highly dependent on the material of the composite structure, the thickness of the laminate layers, and the material properties of the structure. Repeatable responses for complex composite structures are possible if the excitation signal is properly selected.
With respect to probe arrays for use with the pitch-catch method, various geometries have been proposed for the arrangement of transducers and the some prior art systems disclose multiple transducers in one probe device in which not all of the transducers are switched on or involved in nondestructive testing at the same time.
Commercially available Lamb wave-based testing apparati, however, are typically limited to two-dimensional location of defects and are conventionally not able to detect the presence of some types of defects, e.g., depth of delamination in carbon fiber composites. Further, when the presence of defects are detected, the such apparati do not provide a user-friendly way of identifying the various types of defects and do not enable an operator to differentiate between defects at different depths and defects of different types.
Thus, what is needed is a method and apparatus for Lamb wave-based non-destructive testing of composite structures which distinguishes between a variety of defects types not previously distinguishable and which provides a more user-friendly presentation of test results. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.